Heat driven heat pumps which use solid adsorbent beds to adsorb and desorb a refrigerant are known in the art. These solid adsorbent beds adsorb and desorb a refrigerant vapor in response to changes in the temperature of the adsorbent. One common example of such solid adsorbent material is a molecular sieve, such as a zeolite. Other materials which exhibit this phenomena are silica gel, alumina, activated charcoal, and some metal salts. Most any liquid which can be vaporized can be employed as the refrigerant. Water is commonly used as a refrigerant when zeolite is the solid adsorbent.
In the operation of sorption cooling systems, generally there are two or more solid beds containing a solid adsorbent. The solid adsorbent beds desorb refrigerant when heated and adsorb refrigerant vapor when cooled. In this manner the beds can be used to drive the refrigerant around a heat pump system to heat or cool another fluid such as a process stream or to provide space heating or cooling. In the heat pump system, commonly referred to as the heat pump loop, or a sorption refrigeration circuit, the refrigerant is desorbed from a first bed as it is heated to drive the refrigerant out of the first bed and the refrigerant vapor is conveyed to a condenser. In the condenser, the refrigerant vapor is cooled and condensed. The refrigerant condensate is then expanded to a lower pressure through an expansion valve and the low pressure condensate passes to an evaporator where the low pressure condensate is heat exchanged with the process stream or space to be conditioned to revaporize the condensate. When further heating no longer produces desorbed refrigerant from the first bed, the first bed is isolated and allowed to return to the adsorption conditions. When the adsorption conditions are established in the first bed, the refrigerant vapor from the evaporator is reintroduced to the first bed to complete the cycle. Generally two or more solid adsorbent beds are employed in a typical cycle wherein one bed is heated during the desorption stroke and the other bed is cooled during the adsorption stroke. The time for the completion of a fall cycle of adsorption and desorption is known as the "cycle time." The heating and cooling steps are reversed when the beds reach the desired upper and lower temperature limits of the adsorption cycle. The upper and lower temperatures will vary depending upon the selection of the refrigerant fluid and the adsorbent. The efficiency in cooling is called the "coefficient of performance" (COP) and is generally the ratio of the cooling effect divided by the heat input. The thermodynamic aspects of developing a zeolite-water adsorption refrigeration unit are well known. An article entitled, "Thermodynamic Analysis of a Solar Zeolite Refrigeration System," by S. Chang and J. A. Roux, which appeared in the Journal of Solar Energy Engineering, August 1985, Volume 107, pages 189-194, provides a discussion of the main parameters, including adsorber properties.
U.S. Pat. No. 4,610,148 to Shelton discloses a heat driven heat pump system wherein a temperature gradient is established lengthwise in the solid adsorbent bed in order to establish a thermal wave in the bed. As a heat transfer fluid is circulated through the system by a reversible pumping means, the beds are cycled between an upper and a lower operating temperature, creating the thermal wave within the bed of solid adsorbent. The heat transfer fluid always flows serially from a heater through a bed heat exchanger heating that bed while cooling the heat transfer fluid. Then the heat transfer fluid is passed through the cooling heat exchanger to further cool the heat transfer fluid, and the further cooled heat transfer fluid is passed through the other bed heat exchanger to cool that bed while heating the heat transfer fluid. Finally, the thus heated heat transfer fluid is returned to the heater to raise the heat transfer fluid to the original temperature. The solid adsorbent beds are constructed of one or more tubes through which the heat transfer fluid is passed and around which the solid adsorbent is held by a housing shell. In a similar apparatus for use with an ammonia refrigerant, U.S. Pat. No. 5,388,637 discloses the use of a finned tube matrix comprising a bonded activated carbon and a resol bonder tightly adjoined to the fins of the tube to provide high rates of heat transfer between the refrigerant and the heat transfer fluid.
Some thermodynamic processes for cooling and heating by adsorption of a refrigerating fluid on a solid adsorbent use zeolite and other sorption materials such as activated carbon and silica gel. In these processes, the thermal energy from adsorbing zeolite in one zone is used to heat desorbing zeolite located in another zone. U.S. Pat. No. 4,138,850 relates to a system for solar heat utilization employing a solid zeolite adsorbent mixed with a binder, pressed, and sintered into divider panels and hermetically sealed in containers. The U.S. Pat. No. 4,637,218 to Tchernev relates to a heat pump system using zeolites as the solid adsorbent and water as the refrigerant wherein the zeolite is sliced into bricks or pressed into a desired configuration to establish an hermetically sealed space and thereby set up the propagation of a temperature front, or thermal wave through the adsorbent bed. The bricks used in U.S. Pat. No. 4,637,218 are preferably not more than 10 mm in thickness. U.S. Pat. No. 5,477,705 discloses an apparatus for refrigeration employing a compartmentalized reactor and alternate circulation of hot and cold fluids to create a thermal wave which passes through the compartments containing a solid adsorbent to desorb and adsorb a refrigerant.
U.S. Pat. No. 4,548,046 relates to an apparatus for cooling or heating by adsorption of a refrigerating fluid on a solid adsorbent. The operations employ a plurality of tubes provided with parallel radial fins, the spaces between which are filled or covered with solid adsorbent such as Zeolite 13X located on the outside of the tubes.
In an article by Aittomaecki, A. and Haerkoenen, M., titled, "Internal Regeneration of the Adsorption Process," and presented at the Solid Sorption Refrigeration Symposium--Paris, France, Nov. 18-20, 1992, the authors indicate that there is a draw back to the Tchernev/Shelton cycle which creates a thermal wave in the bed traveling in the direction of the flow of the heat transfer fluid. Aittomaecki et al. notes that the cycle time must be short enough to maintain the operation temperatures of the outflowing fluids at the desired level; however, short cycles decrease the net adsorption and lead to a decrease in the COP of the basic process. Thus, there are finite limits to the thermal wave processes which must have a cycle time long enough to maintain adsorbent regeneration efficiency, but short enough to maintain the overall COP.
U.S. Pat. No. 5,279,359 to Erickson discloses an apparatus and a process for sorption heat pumping using a multiplicity of intermittent cyclic triplex sorption modules. The cyclic triplex sorption modules comprise hermetically sealed tubes, each of which contains at least two solid sorbents and is filled with a refrigerant. The preferred refrigerant is ammonia and the solid sorbents are salts such as BaCl.sub.2, SiCl.sub.2, Ca Cl.sub.2, MnCl.sub.2, FeCl.sub.2 and SiBr.sub.2.
U.S. Pat. No. 4,660,629 to Maier-Laxhuber et al. discloses a continuous adsorption cooling device comprising a plurality of adsorption containers filled with adsorbent wherein the adsorption containers are rotated through flow segments which form passageways for a heat carrier stream. The adsorption containers contain an adsorption substance from which an operating substance is extracted by absorbing heat from a heat carrier flow and into which the operating substance is readsorbed, emitting heat to a further heat carrier flow.
Refining processes which are highly thermally integrated represent a significant opportunity for employing sorption cooling systems to recover heat in one portion of a process and provide cooling in another portion of the process. Applications of sorption cooling to such large energy consuming processes have been limited for the reason that variations in temperature in a highly thermally integrated process can not be tolerated, particularly where those variations in temperature may cause a process stream to condense thereby resulting in an upset of the operation of the process.
Prior methods of using zeolite adsorbents in devices for cooling or heating by adsorption of a refrigerating fluid on a solid adsorbent have been inefficient and difficult to prepare. Those methods of preparation included cutting natural rock into thin bricks and mounting these bricks on to heat exchange surfaces or casting powdered zeolites and mixtures thereof with clays into panels or slabs for direct contact with fluids. Prior devices have sought to minimize heat transfer losses in systems for sorption cooling by employing flat containers filled with adsorbent suspended in heat carrier streams or with slabs of adsorbent wired or mounted next to heat transfer surfaces. In one case, finned tubing was employed as a support for a resin bonded adsorbent to provide more thermal conductivity to the adsorbent. Many of these devices incorporated further flow enhancers such as sorbate conduits, weirs, valves, and wicks to establish maximum contact of the operating fluid and the adsorbent with heat exchange surfaces. These devices are limited by their ability to maintain the regenerating efficiency of the adsorbent and their ability to provide continuous heat transfer between an external process and the sorption cooling system.
It is the object of the instant invention to provide an improved sorption cooling system for use in the process industries which is not limited by the regeneration efficiency of the adsorbent.
Unlike simple space or water heating systems wherein some variation in delivered temperature is tolerated or even expected, in the process industry variation in temperature of process streams can result in unacceptable variations in product composition or yield. Therefore, methods are sought which provide low cost heat recovery with sorption cooling systems integrated in such a manner to provide an overall stable, continuous operation.